In known diaphragm pumps of this generic type (DE-AS 1 034 030, DE-OS 25 26 925), the diaphragm is pretensioned with a compression spring. The compression spring is arranged either in the delivery chamber of the diaphragm pump or in its hydraulic chamber, and in such a manner that is assists the movement of the diaphragm in the direction of the suction stroke.
Since it is only a weak compression spring that is concerned here, it is also only a relatively light pretensioning of the diaphragm that is provided. This has the result that the diaphragm positional control is still not satisfactorily provided in every situation. Therefore, additional design elements are necessary for diaphragm positional control, which naturally complicate the structure of the diaphragm pump and thus make it more expensive.
In addition to this there is the fact that due to the slight pretensioning exercised by the relatively weak spring, gas formation in the hydraulic chamber is not effectively prevented during the suction stroke. Thus, because of the still present gas formation in the hydraulic chamber, the overall suction performance of the known diaphragm pumps is limited.
In diaphragm pumps of this generic type, their start-up reliability is of great significance. In modern diaphragm pumps, the lack of start-up reliability can be regarded as a distinct disadvantage. This is only rectified if additional design devices are present, although these bring additional costs. It is therefore desirable with such diaphragm pumps to have sufficient start-up reliability so as to ensure that—due to continuous internal leakage—when the pump is at a standstill, the diaphragm will still not move in the direction of the compression stroke even when there is a vacuum in the delivery chamber.